Many of the prior art refrigeration systems are provided with an arrangement for defrosting the evaporator when the latter becomes covered with ice, rendering it inefficient as a cooler. One defrosting arrangement in a compressor type refrigeration system such as disclosed, for example, by the patent to Hoesel U.S. Pat. No. 2,281,770, comprises supplying the refrigerant directly to the evaporator from the compressor through an arrangement which heats the fluid while bypassing the condenser. In this way, the heated refrigerant causes melting of accumulated frost on the evaporator surface. Such defrosting system depends entirely on the operation of the compressor. In a second refrigeration system wherein a compressor is utilized, defrosting of the evaporator is effected by filling a storage device located below the evaporator with cooled liquid refrigerant from the condenser during the cooling cycle. When defrosting is desired, flow of refrigerant from the compressor to the evaporator is terminated, the liquid refrigerant in the storage device is heated to boil off and the vaporized fluid permitted to flow upward to the evaporator in heat exchange relation with the frost accumulated thereon to melt it, the vapor being thus liquified and returned to the storage device by gravity for reheating. Such a system is disclosed in the patent to Lewis U.S. Pat. No. 2,630,685.
In still another compression refrigeration system as disclosed in the patent to Powers et al U.S. Pat. No. 2,628,479, the cooled liquid refrigerant from the condenser is accumulated in a storage tank from which it is pumped directly to the evaporator unit by a liquid pump during the cooling period or through a heat exchanger first, before applying it to the evaporator unit during the defrosting period. The partially vaporized refrigerant from the evaporator unit returns to the storage tank to build up the pressure therein to maintain the refrigerant in the liquid phase.
All of the above prior art systems depend for their refrigerating operation on the standard compressor. Additionally, in U.S. Pat. No. 2,281,770, the compressor is relied upon for feeding hot refrigerant to the evaporator during the defrost cycle. The others of the above mentioned prior art patents, while they do not rely upon the operation of the compressor to furnish hot refrigerant to the evaporator during the defrost cycle, have the disadvantage that a considerable amount of time is required to heat up the cold liquid refrigerant stored in a storage receiver, as it comes from the condenser, before passing it to the evaporator to defrost the latter. Also, when the cooling cycle of the system is commenced, after defrosting the surface of the evaporator unit, a considerable time elapses before the liquid refrigerant is cooled down sufficiently by operation of the compressor system to cause the evaporator to reach the desired low temperature.